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Linux I/O tuning: CFQ vs. deadline

From: Greg Smith <greg(at)2ndquadrant(dot)com>
To: pgsql-performance(at)postgresql(dot)org
Subject: Linux I/O tuning: CFQ vs. deadline
Date: 2010-02-08 09:45:10
Message-ID: (view raw or whole thread)
Lists: pgsql-performance
Recently I've made a number of unsubstantiated claims that the deadline 
scheduler on Linux does bad things compared to CFQ when running 
real-world mixed I/O database tests.  Unfortunately every time I do one 
of these I end up unable to release the results due to client 
confidentiality issues.  However, I do keep an eye out for people who 
run into the same issues in public benchmarks, and I just found one:

The problem analyzed in the "Deadline considered harmful" section looks 
exactly like what I run into:  deadline just does some bad things when 
the I/O workload gets complicated.  And the conclusion reached there, 
"the deadline scheduler did not have advantages in any of our test 
cases", has been my conclusion for every round of pgbench-based testing 
I've done too.  In that case, the specific issue is that reads get 
blocked badly when checkpoint writes are doing heavier work; you can see 
the read I/O numbers reported by "vmstat 1" go completely to zero for a 
second or more when it happens.  That can happen with CFQ, too, but it 
consistently seems more likely to occur with deadline.

Greg Smith    2ndQuadrant   Baltimore, MD
PostgreSQL Training, Services and Support


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Next:From: Albe LaurenzDate: 2010-02-08 14:57:25
Subject: Re: Linux I/O tuning: CFQ vs. deadline
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